Question

what is the realation between enthalpy and spontaneity​

Answer 1

Enthalpy isn't the only responsible for determining the spontaneity of a reaction, because reactions like melting of ice and evaporation of rain water are endothermic reactions but proceed spontaneously. It has to be associated with entropy and this is expressed in the definition of Gibbs free energy, that is ΔG = ΔH - T ΔS at standard temperature and pressure.

The sign of ΔG depends on the signs of the changes in Enthalpy (ΔH) and entropy (ΔS), as well as on the absolute temperature (T, in kelvin).

When ΔG is negative, a process or chemical proceeds spontaneously in the forward direction.

When ΔS is positive and ΔH is negative, a process is always spontaneous.

When ΔS is positive and ΔH is positive, the relative magnitudes of ΔS and ΔH determine if the reaction is spontaneous. High temperatures make the reaction more favorable.

When ΔS is negative and ΔH is negative, the relative magnitudes of ΔS and ΔH determine if the reaction is spontaneous. Low temperatures make the reaction more favorable.

When ΔS is negative and ΔH is positive, a process is not spontaneous at any temperature

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Answer 2

Answer:

As we can see from our equation, Gibbs free energy is calculated from the changes in enthalpy and entropy, as well as the temperature at which the reaction is carried out at. Let us quickly remind ourselves about these important thermodynamic quantities.

Enthalpy (H) is a measure of how much energy is released or absorbed during a chemical reaction. Energy, in the form of heat, is released in an exothermic reaction, and the change in enthalpy is negative, -H. On the other hand, energy, in the form of heat, is absorbed in an endothermic reaction, and this time the change in enthalpy is positive, +H.

The second property is the measure of entropy (S), which is a measure of disorder or randomness in the system. In nature, a messy room is far more favored than a neat, ordered room, and when disorder increases, we have +S.

Now it turns out that for a spontaneous reaction, the sign of Delta G must be negative. Knowing this, what effect does enthalpy and entropy have on spontaneity? Let us first look at enthalpy: an exothermic reaction is more likely to be spontaneous; if Delta H is negative, this makes it more likely that Delta G will also be negative and, therefore, spontaneous. And now, let's look at entropy: if a reaction causes an increase in randomness of the system, it is more likely to be spontaneous; a positive Delta S term will also mean that Delta G is more likely to be negative.

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